The prior art teaches two principle methods for making alkali metal and alkaline earth metal ferrates. One is by the electrolysis of an alkali metal hydroxide anolyte solution in an electrolytic cell having an iron containing anode. The second process involves the reaction of inorganic hypochlorite with iron containing compounds in an aqueous alkaline solution to produce said ferrate. Regardless of which process is used, the product is isolated by precipitation from a chilled alkaline mother liquor. In practice, the salt normally produced, by either method, is sodium ferrate. Where potassium, barium, and other ferrate salts are desired, these are most usually produced from sodium ferrate by methods utilizing secondary precipitation reactions. Techniques for doing this are known in the art.
Where sodium ferrate is produced, the crude salt precipitate normally contains between about 60 and about 80 percent Na.sub.2 FeO.sub.4, between about 25 and about 10 percent impurities comprising both unconsumed reactants and reaction products such as NaCl, NaOH, NaClO.sub.3, NaOCl, and between about 15 and about 10 percent residual water.
However, it is found that sodium ferrate containing such levels of water and other impurities is unstable and tends to degrade very quickly. This lack of stability is primarily due to the reduction of sodium ferrate in the retained water to form ferric hydroxide, sodium hydroxide and oxygen. Further, the rate of decomposition is often accelerated by the residual impurities such as alkali hydroxides or carbonates which either catalyze the degradative reactions and/or absorb additional water from the surrounding atmosphere. Consequently, in preparing a stable purified solid ferrate product, it is necessary that the water and other impurities therein be removed as quickly and completely as possible.
Typically, one or more organic solvents are used to remove the water, excess reactants and reaction products from the crude ferrate product. Solvents useful for this purpose must meet several requirements. For example, they should be capable of extracting and so removing the retained water and other impurities while dissolving relatively little or none of the precipitated ferrate itself. In addition, the solvent must have substantial oxidation resistance so that any small residues left in the ferrate prior to drying will not cause degradation of the product. As a practical matter, no one organic solvent possess all of these properties and it is common to use a combination of solvents, in sequence, to provide all of these capabilities.
For example, Audette and Quail, in "Potassium, Rubidium, Cesium and Barium Ferrates(VI). Preparations, Infrared Spectra, and Magnetic Susceptibilities", Inorganic Chemistry, Volume 11, No. 8, pages 1904-1908 (1972), disclose a procedure in which the crude ferrate product was first washed six times with chilled alkali metal hydroxide, with rapid filtration between each wash to prevent product decomposition, three times with dry benzene to remove "excess" water and next three times with absolute methanol. The precipitate was then transferred to a centrifuge wherein it was given an additional 15 to 20 washings with absolute methanol. The methanol wash was followed by five washings with anhydrous ether to remove the methanol.
The vacuum filtered, ether damp product was stored overnight in a vacuum dessicator after which it was given an additional 20 washings with absolute methanol and five more washings with anhydrous ether. Only about a 50 percent yield of 99+ percent K.sub.2 FeO.sub.4 was obtained, and the product still had to be stored in a vacuum dessicator to avoid slow decomposition due to traces of water and ether still present on the product. Other procedures in the literature replace the benzene and methanol with solvents such as ethanol, isopropanol or secondary butanol.
However, ferrates are very strong oxidizers and can oxidize many organic compounds in the presence of water very easily. For example, in neutral or acidic aqueous solutions, or at mildly elevated temperatures, primary alcohols are quickly oxidized to their corresponding aldehydes and carboxylic acids, secondary alcohols are oxidized to ketones and many ethers are oxidized to extremely hazardous peroxides, all such reactions resulting in severe degradation of the finished product. Thus, as a practical matter, all of these procedures show significant economic, safety or operational deficiencies which act to severely inhibit the ability to make and utilize ferrate materials.